Built on a mark 1 phase II Sprinter 416 chassis which was manufactured up until 2005 so the first registration of this motorhome is two years after that. A good, workmanlike base with loads of payload. Check that it has a true 5-speed automatic gearbox rather than the 6-speed Sprintshift semi-auto which was prone to problems in some cases.
There were only about 50 Autotrail Grande Frontiers made (it was widely regarded as a failure and production soon ceased) so you would in effect be buying a prototype. The only British A-class ever built I believe and it proved to be no match for the Germans and French.
There are several references to the Grande Frontier on Fun (and other forums). Some owners have been very pleased - others have had major problems with build quality - I recall tales of one owner returning it after 8 weeeks and another after 12 weeks.
I guess one of the biggest issues would be that there cannot be a big stock of spare parts for something that was a commercial failure and therefore never got into a long-term production run (it was in production for about one year I think).
Caveat emptor would be high in my mind if considering this.

We nearly bought one a few years ago but after contacting several owners changed our minds. MANY problems came to light as soon as they were manufactured from electrics to the useless gearbox. Lovely looking van but habitation spares could be a challenge to find.
Most were sold off new and cheap for as low as £38,000 so a bargain could be had by the brave. A local dealer just offloaded an early model for £26500.

I suspect you are thinking of the Swift Bel-Air which was a re-badged Italian Mirage.
So far as I am aware, the Grande Frontier was made in Britain. Pity really as Auto Trail might have been better off re-badging a Euramobil A-class which is made by another subsidiary of the Trigano Group.